There is currently a trend in telecommunication to use Wireless Local Area Network (WLAN) for mobile telephony and for mobile data services. WLAN may be applied in a variety of ways, for telephony and data services, including, but not limited to:                IP Multimedia Subsystem (IMS) access through WLAN: an IMS subscriber may obtain IP connectivity through WLAN, for the purpose of accessing the IMS network; accessing the IMS network is used to request IMS services. The user may thus establish speech calls or other IMS services;        Unlicensed Mobile Access (UMA)/Generic Access Network (GAN): this method entails that a GSM subscriber uses WLAN as access method to a GSM core network, instead of using the GSM Radio Access Network (RAN) for accessing the GSM core network.        
In both of the above cases, the WLAN access would typically be augmented by generic IP transport mechanism like Digital Subscriber Line (DSL) or Cable modem. The above-described methods of applying WLAN have in common that for both signalling and media transfer to and from the mobile terminal, WLAN is used as transport medium. The signalling is carried over the ‘Control Plane’; the media is carried over the ‘User Plane’.
When UMA is applied, the signalling and the media transfer for a UMA-capable GSM terminal is routed through the GSM core network. This entails, amongst others, that the signalling (Control plane) runs between the terminal and an MSC Server and that the media (User plane) runs between the terminal and a Media Gateway (MGW). So the media will, when using UMA, always run through the MGW in the GSM core network of a corresponding GSM operator. However, it is not desirable that the media is transferred through the GSM core network in e.g. the following cases:                using UMA on enterprise premises; employees have mobile phone with UMA access; when these employees establish calls between one another, there is not always a need for the media to run through the GSM core network;        using UMA for city-wide coverage of WLAN; users of a public WLAN network may establish calls with people in close vicinity; the media does not necessarily need to travel through the MGW in this case.        